1. Field of the Invention
This invention relates generally to a device and method for clock recovery and specifically to a device and method for the recovery of a sub-harmonic clock rate from an optical pulse stream.
2. Description of the Prior Art
Optical fibers are often used to transmit binary data encoded upon a string of evenly-spaced optical pulses; this is generally known as a return-to-zero (RZ) format. The pulses are generated by an optical source typically producing at a rate of several gigahertz (GHz). The data is encoded upon them with an amplitude modulator, which allows a pulse to pass to express a binary “one” and which eliminates a pulse to express a “zero.” RZ data therefore consists of a modulated pulse train.
The approximate maximum rate at which electronic data can be generated and converted to optical data is currently between 10 to 20 GHz. An optical fiber, however, is capable of carrying RZ data at much higher rates, if the duration of the optical pulses is sufficiently short. A common technique for increasing the data transmission capacity of a fiber transmission system, therefore, is to time interleave, or multiplex several optical data channels. This procedure is known as optical time division multiplexing (OTDM). The data transmission rate (line rate) of a fiber carrying N OTDM channels is then N times the rate of an individual channel. FIG. 1 illustrates an OTDM data transmitter. In FIG. 1 several individual channels (DATA 1–DATA N) having a common frequency are time interleaved and combined into a single data stream with multiple channels.
A problem at the receiving end is to de-interleave, or demultiplex, the individual constituent channels from the multi-channel data stream. This is a challenging problem because the line rate of the optical data stream may be too high for it to be divided down to the individual channel rate by electronic means.
One approach to the electronic switching speed limitation has been to use “all optical” clock recovery techniques. Generally, these techniques include nonlinear optical interactions in fiber loop mirrors, semiconductor optical amplifiers and self-pulsating diode lasers. These techniques, however, are susceptible to instabilities in the nonlinear media which leads to noisy or unreliable operation. They also require the added expense and complexity of a picosecond laser driven at the local recovered clock frequency.
Another approach has been to employ bootstrapped optical clock recovery and demultiplexing as shown in FIG. 2. In the bootstrapped system a single channel extracted from a demultiplexer is used to generate an error signal to trim the frequency of a local voltage-controlled oscillator. A demultiplexer operates by operating an optical switch at the single channel rate. The effective time window of the switch must be narrow enough to capture a single pulse. When the voltage-controlled oscillator's frequency is different from that of the data stream, a periodic signal will be detected by the photodetector whose frequency is the difference between the two frequencies. A phase detector is generally used to detect these differences. When the frequency difference is zero, the phase detector's output is a signal representing the phase difference between the two signals. This phase difference is used to ensure that the voltage-controlled oscillator's frequency and phase are maintained at the optimum values for extraction of the sampled channel.
Another approach has been to attempt single channel clock recovery using an ultra-fast electro-optic device as shown in FIG. 3. This approach uses a phase-locked loop (PLL) driving a voltage controlled oscillator in a manner similar to the bootstrapped approach, but employs a separate optical switch with monitors a single channel in the data stream to produce an error signal.
Yet another approach has been to employ an optical intensity-based feedback system as shown in FIG. 4. The circuit shown in FIG. 4 achieves single channel clock recovery by using an electro-optic modulator sampling the bit stream as a phase detector. Specifically, the phase difference between the optical switch and the data stream determines whether a pulse falls within the demultiplexer's time window. The average signal from a photodetector monitoring the switch will be zero if the pulses are outside the timing window and maximum if the pulses lie entirely within the window. The signal can be used as an error signal in a PLL to force the voltage-controlled oscillator driving the optical switch to operate at the proper frequency to extract the individual channels.
FIG. 5 shows yet another approach, which is a single-channel, intensity-based clock recovery circuit system employing an electro-absorption modulator (EAM) as reported by Tong et al.
The Tong device employs a balanced photodetector in an intensity-noise-cancelling system. The sampled current in the balanced photodetector system is the difference between the output of a photodetector monitoring the EAM and that of a photodetector monitoring an attenuated portion of the bit stream. The attenuator is adjusted so that the average current output is zero. Any noise due to optical intensity variations in the bit stream is of the same magnitude in both photodetectors and is cancelled out. The output of the balanced photodetector thus varies only due to variations in the phase between the bit stream and EAM's switching window.